Electronic toll collection systems conduct toll transactions electronically using RF communications between a vehicle-mounted transponder (a “tag”) and a stationary toll plaza transceiver (a “reader”). An example of an electronic toll collection system is described in U.S. Pat. No. 6,661,352 issued Dec. 9, 2003 to Tiernay et al., and owned in common with the present application. The contents of U.S. Pat. No. 6,661,352 are hereby incorporated by reference.
In a typical electronic toll collection (ETC) system, the reader broadcasts a wakeup or trigger RF signal. A transponder on a vehicle passing through the broadcast area or zone detects the wakeup or trigger signal and responds with its own RF signal. The transponder responds by sending a response signal containing information stored in memory in the transponder, such as the transponder ID number. The reader receives the response signal and may conduct an electronic toll transaction, such as by debiting a user account associated with the transponder ID number. The reader may then broadcast a programming RF signal to the transponder. The programming signal provides the transponder with updated information for storage in its memory. It may, for example, provide the transponder with a new account balance.
In ETC systems, a number of lane-based reader antennas are deployed, often on an overhead gantry. Each of the lane-based reader antennas has a radiation pattern within the roadway. Similarly, each of the transponders has a radiation pattern. As a vehicle-borne transponder traverses a toll area, the respective radiation patterns of the reader antennas and the transponder define a capture zone in which the reader and transponder are capable of sending and receiving RF communications with each other.
In existing ETC systems, the timing of operations and decision-making are partly defined by the size of the capture zone. Accordingly, any modifications to transponders that are intended for use in existing ETC systems preferably provide a radiation pattern resulting in a similar size capture zone, even if achieving improvements in power-usage, sensitivity, etc.